Passive fluid recovery system

ABSTRACT

A fluid recovery system is adapted for use with a cooling system, such as for use in electronic applications. In one example, an enclosure is configured to contain fluid in both gas and liquid states, wherein the fluid is adapted for use in spray cooling electronic components. A plurality of pick-up ports is defined within the enclosure. In one implementation of the cooling system, an orifice size used in each of the pick-up ports results in withdrawal of fluid from submerged and non-submerged pick-up ports.

BACKGROUND

Many electronic devices are air-cooled, such as by a passive or activecooling system. Passive cooling systems can be as simple as inclusion ofventilation slots in an enclosure containing the device. Active coolingsystems can include a fan, which typically increases the rate of heatremoval.

In more advanced cooling systems, a dielectric fluid can be used toprovide even greater heat removal. Fluid is continuously cycled betweena sprayer, locations wherein the fluid is in contact with components,and a pump. In a stationary system, a sump may collect liquid dielectricfluid from the bottom of an enclosure surrounding the system. However,in a mobile system, both the rate of acceleration and the direction ofacceleration of the enclosure can vary with time. As a result,dielectric fluid does not consistently move to an expected locationwithin the enclosure. Accordingly, recovery of dielectric fluid fromwithin a mobile spray-cooled system presents challenges that have notpreviously been fully resolved.

SUMMARY

A fluid recovery system is adapted for use with a cooling system, suchas for use in electronic applications. In one example, an enclosure isconfigured to contain fluid in both gas and liquid states, wherein thefluid is adapted for use in spray cooling electronic components. Aplurality of pick-up ports is defined within the enclosure. In oneimplementation of the cooling system, an orifice size used in each ofthe pick-up ports results in withdrawal of fluid from ports that aresubmerged in liquid coolant and ports not submerged in liquid coolant.

This Summary is provided to introduce selected topics, which are furtherdescribed below in the Detailed Description. Accordingly, the Summary isnot intended to identify key or essential features of the claimedsubject matter. In particular, the Summary is not to be used as an aidin determining the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to the accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentifies the figure in which the reference number first appears. Theuse of the same reference numbers in different figures indicates similaror identical items.

FIG. 1 is a block diagram illustrating an exemplary passive fluidrecovery system.

FIG. 2 illustrates aspects of pick-up port placement in an exemplaryfluid-cooled system having dimensions consistent with clamshellconstruction.

FIG. 3 illustrates aspects of pick-up port placement in an exemplaryfluid-cooled system having an enclosure dimensions consistent with sixsimilarly sized sides.

FIG. 4 shows a plurality of pick-up ports and a pump, modeled after aresister network as an aid in discussion.

FIG. 5 illustrates an example method by which a passive fluid recoverysystem can be designed.

FIG. 6 illustrates an example method by which a passive fluid recoverysystem can be operated.

DETAILED DESCRIPTION

Overview of the Problem

Spray cooling is a highly efficient method of cooling electronics. Inone example of spray cooling, a dielectric liquid is sprayed onto thecomponents within a fluid-tight enclosure. Some of the liquid lands onthe electronic components and removes waste heat. A portion of the heatis removed from the devices through heating of the liquid. Additionalheat is removed by a phase change of the sprayed liquid, resulting in avapor. Both the liquid and the vapor then pass through a heat exchangerwhere the vapor is condensed and the liquid is sub-cooled below theliquid saturation curve.

In some spray module systems, momentum of the vapor can be utilized tocarry both the vapor and the liquid to a remote heat exchanger. This ispossible in spray modules of small volume, wherein the conduit to theheat exchanger is appropriately designed. Where the flow is primarilymomentum driven, the system will operate in any orientation. In the heatexchanger, the vapor is condensed and the liquid is sub-cooled beforeentering the pump.

In other spray cooling systems, a small volume of liquid is containedwithin an enclosure having a relatively large volume. This results inrelatively small fluid velocities and the separation of fluid into vaporand liquid phases within the enclosure. Condensation on the enclosurewalls further reduces the vapor quality (i.e. the percentage of thefluid in the vapor phase). Low vapor quality and low vapor velocitiestend to counter fluid movement toward the heat exchanger and/or the pumpinlet. As a result, liquid moves freely within the enclosure, and isdriven almost exclusively by inertial forces. In stationary systems, asump may be incorporated in the bottom of the enclosure to collect theliquid. In mobile systems, a variable acceleration vector can drive thefluid in any direction. This presents the problem of removing a smallamount of liquid in a relatively large volume defined by the enclosure.

Systems that allow operation in any orientation and under a variableacceleration vector can be referred to as attitude independent (AI).Such systems operate independently of the attitude of the vehicle withinwhich the system is mounted. In a mobile system, acceleration vectorstend to pool fluid in locations within the enclosure that include one ormore corners. Liquid can be removed from such a system using mechanicalvalves that are actuated in response to inertia, or using solenoidvalves activated by capacitive fluid sensors. In both systems havingeither type of valve, a fluid pick-up port may be located at variouslocations within the main enclosure but are typically located near eachcorner of the enclosure.

However, by incorporating valves into each corner of a system, a largenumber of potential failure points are created. Moreover, the cost andpower draw of the active AI system are also of concern. Accordingly,design of a passive fluid recovery system would obviate some of theseconcerns.

Example System

FIG. 1 is a block diagram illustrating an exemplary passive fluidrecovery system 100. A clamshell enclosure 102 includes similar front104 and back (not shown) faces separated by a distance that is less thaneither dimension of the front and/or back faces. Within the clamshellenclosure 102 a spray module 106 is configured to spray coolant at oneor more ‘load boards’ 108, which can be configured as electronic circuitcards including one or more heat-producing electronic devices. A heater110 is sized to produce heat required to increase vapor quality atstart-up, prior to turning on the load board 108. That is, the heater110 will increase the percentage of the fluid that is in the vapor (i.e.gas) phase. In one implementation, the system 100 is configured togenerate a vapor quality resulting in a pressure of 0.23 psi (pounds persquare inch) for operation. In normal running mode, heat generated bythe electronics creates the needed vapor. During startup, before theelectronics are powered, another means of generating vapor may beneeded. Accordingly, heaters 110 that are initially submerged in aliquid pool in the enclosure 102 can be used for this purpose.

Fluid, in both liquid and gas phases, is removed from the enclosure 102through pick-up ports 112-126, and is transferred via tubing or pipes128, 130 to a heat exchanger 132. The heat exchanger 132 uses in-comingair 134 to remove heat from fluid within the heat exchanger. Heated airis exhausted at 136, 138. Removal of heat while in the heat exchanger132 fully transforms vapor components of the fluid removed from theenclosure 102 into liquid, and cools the liquid. The fluid is thenremoved from the heat exchanger 132 via pipe 140 and passed into thepump 142, which increases the pressure of fluid leaving the pump at 144,for introduction into the spray module 106. Note that in a compound heatexchanger, each of a plurality of channels 146 may be associated with asingle pick-up port orifice. Use of such a compound heat exchanger tendsto isolate either liquid or vapor in each channel. This tends to preventvapor from passing through the heat exchanger without condensing intoliquid.

When electronic components (e.g. components on load board 108) are spraycooled (such as by spray module 106), only a portion of the sprayedliquid is vaporized. The percentage of the fluid vaporized can rangefrom zero percent in a relatively large enclosure having relatively lowpower consumption, to approximately eighty percent or more in relativelysmall enclosures having relatively high power consumption. To maintainthe system in an operative state, fluid comprising both the liquid andvapor states is generally removed from the enclosure at the same ratethat fluid is introduced into the system via the spray module atomizers106—a rate that is governed in part by the power consumption of thesystem. Because fluid, in both the liquid and vapor states, isdistributed within the enclosure 102 in part due to an accelerationvector that changes in both direction and magnitude with time, aplurality of pick-up ports 112-126 are distributed within the enclosure,thereby providing a number of locations from which fluid can be removed.

The distribution of the pick-up ports 112-126, and their diameters,control aspects of withdrawing a satisfactory quantity of both liquidand vapor, simultaneously, for cooling by the heat exchanger 132 andreturn to the enclosure 102 via the spray module 106. Because of themanner in which liquid and vapor fluid are distributed within theenclosure 102, more ports are exposed to vapor than are exposed toliquid. An additional impediment to fluid removal is that those portsthat are exposed to liquid may be at a point lower than the pump inlet,and therefore have a hydrostatic head pressure component to overcome.

Accordingly, a preferred distribution of pick-up ports 112-126 willprovide a passive attitude independent (AI) solution by addressing atleast two important aspects. First, the distribution of pick-up portswithin the enclosure should minimize variance in a number of portssubmerged in liquid. That is, changes in the attitude of the enclosureshould result in minimal change in the number of liquid-submergedpick-up ports. Secondly, the diameter used for the pick-up ports shouldresult in sufficient fluid entering both those ports submerged in liquidand those in vapor, so that the total quantity of fluid removed isroughly equal to that needed to supply the spray module and remove heatgenerated by the load. Additionally, since a majority of the pick-upports will not be exposed to liquid, the heat exchanger 132 must be ableto condense vapor fully, thereby preventing vapor from reaching thepump.

An implementation of the system 100 optionally includes one or morewicks 146, 148, typically located in a fluid path between one or morepick-up ports 112-126 and the spray module 106. The wicks are configuredto remove and hold foreign fluid, such as water, air, etc. A wick can beconfigured in a number of ways; e.g., a fibrous material trapped in awidened area of the pipe (e.g. 128, 130) can be used to catch and retainforeign material. Thus, a wetted wicking material transports liquid,while preventing air or other non-condensable gasses from passing due tosurface tension of the liquid present in the wick. Accordingly, a wicktransports fluid, while essentially removing undesirable gasses from thesystem.

In some cases, it may be desirable to transport some or all of thecaptured noncondensable gasses back into the system at another point inthe system, e.g. at the pump outlet. In other cases, it may be desirableto vent these gasses from the system altogether. In either case, anactive or passive means for noncondensable gas transport can beadvantageous.

Removing Fluid from the System

Liquid in the enclosure can be accessed in any orientation if there isat least one pickup near each corner. If one pickup is placed at thevertex of each corner, the number of pickups covered by liquid can varygreatly with orientation. For example, if the acceleration vector isnormal to the bottom face of the enclosure, four of the eight pickupswill be covered with liquid. Conversely, if the acceleration vector isparallel to a line through the enclosure centroid and a corner, only oneof the eight pickups will be covered with liquid. The variation inoperation from 12.5% coverage of pick-up ports by liquid (as opposed tovapor) to 50% coverage would greatly complicate the fluid controlscheme. In contrast to this example, the examples of FIGS. 2 and 3provide a distribution of pick-up ports wherein the percentage of portscovered by liquid is less variable when an attitude of the enclosure isaltered with respect to gravity and/or acceleration.

FIGS. 2 and 3 show two implementations of pick-up port distributionwithin an enclosure that show a percentage of the pick-up ports coveredwith liquid is relatively constant, despite changes in orientations withrespect to gravity and/or an additional acceleration vector, perhapshaving changing direction and/or magnitude.

Referring to FIG. 2, aspects of a passive fluid recovery system 200 areshown. The system 200 is configured to include a clamshell typeenclosure 102, wherein two pick-up ports are defined in each corner ofboth the front and back sides of the enclosure, resulting in 16 totalpick-up ports 112-126. No pick-up ports are defined in the other foursides of the enclosure. In the enclosure 102 of FIG. 2, the variation inliquid coverage ranges from 19% to 50% of the pick-up ports; conversely50% to 81% of the pick-up ports are covered by vapor (gas). However, ifthe enclosure is constructed so that it is very unlikely that theacceleration vector will be normal to the two large sides of theenclosure, the variation in liquid coverage may be confined to between19% and 25%. The enclosure can be constructed in a manner that preventsacceleration forces from being normal to the large sides by altering theinternal geometry of the large sides (e.g. curving them, slanting them,ramping them or introducing a slope, etc.), thereby making thecombination of these vectors very unlikely to combine to create anacceleration vector normal to the larger faces.

Referring to FIG. 3, aspects of a passive fluid recovery system 300 areshown. The system 300 is configured to include an enclosure having sixsimilarly-sized sides 302, one of which is shown in plan view and fourare seen in cross-section. Each side 302 of the enclosure 300 includesone pick-up port located near each corner. In the example, side 302shown, pick-up ports 304-310 are representative of the 24 pick-up portsincluded in the six sides of the system 300.

Because of the configuration of the system 300, the variation in liquidcoverage ranges from 12.5% to 16.7% of the pick-up ports. This can beunderstood by realizing that if the fluid pooled entirely in one corner,three pick-up ports, out of 24 total, would be entirely covered,resulting in 12.5% of the pick-up ports covered by liquid. However, ifthe liquid is thinly distributed to cover an inside surface of one sideof the enclosure, then 4 of the 24 pick-up ports will be covered withfluid, resulting in 16.7% of the pick-up ports being covered by liquid.

FIG. 3 shows an example of an enclosure constructed to essentiallyprevent an acceleration vector normal to a side of the enclosure.Interior surfaces of enclosure sidewalls 322-328 are curved to directliquid to accumulate in the corners formed by intersection of threesidewalls. Exterior surfaces can be either curved or planar. Because theliquid is so directed, the variation in liquid coverage of pick-up portsmay be confined within a narrow range. For example, in the system 300 ofFIG. 3, it is typical that between 19% and 25% of the pick-up ports arecovered by liquid at any time. The curved sides used to construct theenclosure prevent a combination of gravity and acceleration forces frombeing normal to the sides. While curved concave interior surfaces322-328 are shown, convex sides, round sides, slanting side and sides orother configurations may also be used to reduce variation in thepercentage of pick-up ports covered by liquid. Also, while the interiorsurfaces 322-328 are depicted having a curvature over the entire wall, awall having both orthogonal and curved portions may be more easilyassembled. Thus, an implementation having an enclosure defined by atleast one non-planar wall (e.g. a curved wall) or one non-orthogonalwall (e.g. a wall slanted with respect to other walls) can reduce thevariance in the number of pick-up ports covered by liquid as a functionof an acceleration and/or gravity vector.

Mass Flow Rate Calculation

FIG. 4 shows a plurality of pick-up ports 112-126, 402-416 and a pump142, modeled as a resister network 400, for purposes of discussion ofthe fluid metering problem. The fluid metering problem involves a numberof aspects, although two are of primary importance. First, thepercentage of pick-up ports that are covered in liquid is important,since fluctuation in the percentage tends to result in fluctuation inthe amount of fluid removed from the enclosure. Second, the diameter ofthe pick-up ports is important, and is a function of a selected pump,the rate at which fluid is to removed from the enclosure and otherfactors. In particular, for a given pump, too small a diameter willresult in too little fluid being removed from the system. Conversely, adiameter that is too large will result in too large a volume of vapor,and too little liquid, resulting in removal of too little fluid overall.Accordingly, a preferred pick-up port diameter is between theseextremes. Referring to the resister network 400, a resistance value 418associated with each of the pick-up ports 112-126, 402-416 is partlyrelated to whether the pick-up port is covered by liquid or gas, andpartly related to the diameter of the pick-up port.

Because fluctuation in the number of pick-up ports covered by liquid isa factor in the fluid metering problem, reducing the variation in thepercentage of the pick-up ports that are covered by liquid simplifiesfinding a solution of the fluid metering problem. The mass flow ratethrough each path is a function of resistance. The resistance is in turna function of fluid density. The dependence of resistance on density canbe used to manipulate the flow rate of each phase (liquid and gas)through the network. Restrictive orifices can be added to each path tocounteract the hydrostatic head and balance the flow.

The mass flow rate through a path associated with any pick-up port is afunction of the total pressure drop between the enclosure and the pumpinlet. That is: {dot over (m)}=ƒ(ΔP_(total)). The total pressure drop iscomposed of the pressure drops of the individual components in thepickup path: ΔP_(total)=ΔP_(h)+ΔP_(o)+ΔP_(hx)

Where:

-   ΔP_(h)=hydrostatic head-   ΔP_(o)=pressure drop across orifice-   ΔP_(hx)=pressure drop across other system components, primarily heat    exchanger

Note that in FIGS. 2 and 3, the hydrostatic head is related to thedistance ( dz) between the pump inlet 202, 318 and a fluid level 206,312 within the enclosure. For example, within the enclosure of eachsystem 200, 300, there is a region of liquid 210, 316 and a region ofvapor 208, 214. A surface 206, 312 divides the two, and is dependent onthe attitude of the system. Thus, the hydrostatic head is the pressurerequired to overcome the distance between the liquid surface 206, 312and a parallel line 204, 320 passing through the pump inlet 202, 318.

Thus, hydrostatic head is proportional to the density of the fluid andthe distance between the surface (e.g. 206, 312 of FIGS. 2 and 3,respectively) and the pump inlet 202, 318. Thus, hydrostatic head can bemodeled as ΔP_(h)=ρ( g· d) Where:

-   ρ=fluid density-   g=acceleration vector-   d=displacement vector from pickup to pump inlet

Mass flow rate ({dot over (m)}) through an orifice varies as the squareroot of the density of the fluid. For example:

$\overset{.}{m} = { {C_{d}A\sqrt{2\Delta\; P_{o}\rho}}\Rightarrow{\Delta\; P_{o}}  = {( \frac{1}{C_{d}A} )^{2}\frac{{\overset{.}{m}}^{2}}{2\rho}}}$Where:

-   C_(d)=orifice coefficient-   A=orifice area

Mass flow rates through other restrictions vary as the square root ofthe density of the fluid. Accordingly:

$\overset{.}{m} =  {k\sqrt{2\Delta\; P_{hx}\rho}}\Rightarrow{\Delta\;{P_{hx}( \frac{1}{k} )}^{2}\frac{{\overset{.}{m}}^{2}}{2\rho}} $Where:

-   κ=loss coefficient of system

The total pressure drop through a pickup channel is the sum of theindividual pressure drops.

${\Delta\; P_{total}} = {{{\Delta\; P_{h}} + {\Delta\; P_{o}} + {\Delta\; P_{hx}}} = {{\rho( {\overset{\_}{g} \cdot \overset{\_}{d}} )} + {( \frac{1}{C_{d}A} )^{2}\frac{{\overset{.}{m}}^{2}}{2\rho}} + {( \frac{1}{k} )^{2}\frac{{\overset{.}{m}}^{2}}{2\rho}}}}$Solving for {dot over (m)} yields:

$\overset{.}{m} = \sqrt{\frac{2{\rho( {{\Delta\; P_{total}} - {\rho\;{\overset{\_}{g} \cdot \overset{\_}{d}}}} )}}{\lbrack {( \frac{1}{C_{d}A} )^{2} + ( \frac{1}{k} )^{2}} \rbrack}}$

C_(d)A, k, and z (where z is distance of a pick-up port above or belowthe pump) are design controllable parameters. The term ΔP_(total) is theonly variable that is controllable during operation. The value ofΔP_(total) is controlled by pump speed. The pick-up port position (d₁)is predetermined by the pick-up port placement, wherein the placement isconfigured to lower variability in a number of orifices submerged inliquid. Therefore, a design goal is to choose C_(d)A and k so that theproper mass flow rates of liquid and vapor are achieved within theoperating range of ΔP_(total) of the pump. In a preferred system design,k is much smaller than C_(d)A and can be ignored. For ports in vapor,the term ρ g· d is several orders of magnitude less than the ΔP_(total)or ρ g· d associated with ports immersed in liquid, and can be ignoredfor simplification.

For a system (e.g. system 100 or 200 of FIGS. 1 and 2, respectively)with four ports submerged in liquid and twelve ports in vapor, the vapormass flow rate ({dot over (m)}_(v)), the liquid mass flow rate ({dotover (m)}_(l)), and the total mass flow rate ({dot over (m)}_(total))are given as:

$\begin{matrix}{{\overset{.}{m}}_{v} = {\sum\limits_{1}^{12}\sqrt{\frac{2{\rho\;}_{v}( {\Delta\; P_{total}} )}{( \frac{1}{C_{d}A} )^{2}}}}} \\{= {12C_{d}A\sqrt{2{\rho_{v}( {\Delta\; P_{total}} )}}}} \\{{\overset{.}{m}}_{l} = {\sum\limits_{1}^{4}\sqrt{\frac{2{\rho_{l}( {{\Delta\; P_{total}} - {\rho_{l}{\overset{\_}{g} \cdot {\overset{\_}{d}}_{i}}}} )}}{( \frac{1}{C_{d}A} )^{2}}}}} \\{= {C_{d}A{\overset{4}{\sum\limits_{1}}\sqrt{2{\rho_{l}( {{\Delta\; P_{total}} - {\rho_{l}{\overset{\_}{g} \cdot {\overset{\_}{d}}_{i}}}} )}}}}} \\{{\overset{.}{m}}_{total} = {{\overset{.}{m}}_{v} + {\overset{.}{m}}_{l}}}\end{matrix}$

The relation can be simplified further if the hydrostatic head term (ρg· d) is considered constant between all pick-up ports submerged inliquid. This is a valid assumption because the free surface of theliquid 206, 312 in the enclosure is at a constant distance from the pumpinlet 202, 318 in the direction of g as illustrated in FIGS. 2 and 3.With this simplification, the mass flow rates become:

$\begin{matrix}{{\overset{.}{m}}_{v} = {12C_{d}A\;\sqrt{2{\rho_{v}( {\Delta\; P_{total}} )}}}} \\{{\overset{.}{m}}_{l} = {4C_{d}A\sqrt{2{\rho_{l}( {{\Delta\; P_{total}} - {\rho_{l}{\overset{\_}{g} \cdot {\overset{\_}{d}}_{i}}}} )}}}} \\{{\overset{.}{m}}_{total} = {{12C_{d}A\sqrt{2{\rho_{v}( {\Delta\; P_{total}} )}}} + {4C_{d}A\sqrt{2{\rho_{l}( {{\Delta\; P_{total}} - {\rho_{l}{\overset{\_}{g} \cdot {\overset{\_}{d}}_{i}}}} )}}}}}\end{matrix}$

The total mass flow rate out of the enclosure through the pick-up portsmay approximate the mass flow rate into the enclosure through theatomizers to maintain the system operation. The ratio of {dot over(m)}_(l) to {dot over (m)}_(v) may approximate the ratio generated inthe enclosure. The ratio of liquid to vapor mass flow of the outletstream may approximate the ratio of the mass flow rate through thepick-up ports submerged in liquid to the mass flow rate through thepick-up ports in vapor. Thus, the ratios can be expressed as:

$\begin{matrix}{\frac{{\overset{.}{m}}_{l}}{{\overset{.}{m}}_{v}} = \frac{4C_{d}A\sqrt{2{\rho_{l}( {{\Delta\; P_{total}} - {\rho_{l}{\overset{\_}{g} \cdot {\overset{\_}{d}}_{i}}}} )}}}{12C_{d}A\sqrt{2{\rho_{v}( {\Delta\; P_{total}} )}}}} \\{\frac{{\overset{.}{m}}_{l}}{{\overset{.}{m}}_{v}} = {\frac{1}{3}\sqrt{\frac{\rho_{l}( {{\Delta\; P_{total}} - {\rho_{l}{\overset{\_}{g} \cdot {\overset{\_}{d}}_{i}}}} )}{\rho_{v}( {\Delta\; P_{total}} )}}}}\end{matrix}$

Both {dot over (m)}_(total) and {dot over (m)}_(l)/{dot over (m)}_(v)are determined by the spray configuration and the heat load, andtherefore are known. The term {dot over (m)}_(total) is preset by theatomizer selection and the discharge pressure. The ratio {dot over(m)}_(l)/{dot over (m)}_(v) is a function of the system heat load andthe heat lost through the walls of the enclosure. Any heat lost throughthe walls results in condensation in the enclosure and an increase in{dot over (m)}_(l)/{dot over (m)}_(v).

Implementation

FIG. 5 illustrates an example method 500 by which a passive fluidrecovery system (e.g. systems 100, 200, 300, 400 of FIGS. 1-4) can bedesigned. The method 500 can be implemented in software 502, hardware(e.g. such as by use of an application specific integrated circuit(ASIC)) or in firmware or other environment. In one implementation, themethod uses the system heat load, spray configuration, saturationtemperature, ambient temperature, pickup placement, pickup orificediameter, and enclosure orientation as inputs. The method can output thetotal mass flow rate through the pickup system ({dot over (m)}_(pu)).The output can be compared to the fixed mass flow rate through theatomizers ({dot over (m)}_(atom)). If {dot over (m)}_(pu) is less than{dot over (m)}_(atom), the pickup orifice diameter size is decreased andthe software is rerun. If {dot over (m)}_(pu) is greater than {dot over(m)}_(atom), then the pick-up orifice size is increased and the softwareis rerun. This process may be repeated until the proper orifice size isfound. It should be noted that the system will still operate if thecalculated {dot over (m)}_(pu) is greater than {dot over (m)}_(atom)since the true {dot over (m)}_(pu) is limited to {dot over (m)}_(atom),by the control of the atomizer discharge. The higher calculated {dotover (m)}_(pu) indicates that the change in pressure (ΔP) across thepickup orifices is greater than needed to move the required mass flow.Note that an elevated ΔP may be detrimental, in that it can elevate thesaturation temperature in the enclosure. Therefore, the pickup orificediameter should be as large as possible while maintaining enough {dotover (m)}_(pu).

Referring again to FIG. 5, the method 500 by which a passive fluidrecovery system is designed can be seen in greater detail. At block 504,a system heat load is identified. Referring to FIG. 1, an example of theheat load is the heat produced by the load boards 108. The load boardscan be any heat-producing device; however, the load boards are typicallyone or more electronic assemblies.

At block 506, a fluid flow rate through atomizers ({dot over(m)}_(atom)) associated with the system heat load is calculated. Thefluid flow rate can be a function of the system heat load, identified atblock 504. In an example implementation, the electrical consumption ofthe load board 108 is calculated, measured or otherwise determined,thereby revealing the associated heat load. Having established thesystem heat load, an associated mass flow rate through the atomizers iscalculated. The mass flow rate through the atomizers is the amount offluid that will be required to remove the system heat load. Referring toFIG. 1, the spray module atomizers 106 direct fluid spray (not shown) atthe load boards 108, thereby removing heat from the load boards atapproximately the rate that heat is produced.

At block 508, pick-up orifices are located within an enclosure to makevariation in pick-up orifice liquid coverage substantially attitudeindependent. The enclosure can be configured as a clamshell, wherein twodimensions of the enclosure are substantially greater than a thirddimension. In this case, illustrated in FIGS. 1 and 2, two pick-uporifices (e.g. 112 and 114) may be located near each corner in each ofthe two larger sides of the enclosure 102. As seen in the abovediscussion, such a clamshell enclosure generally limits variation inpick-up orifice liquid coverage to 19% to 25%. The enclosure canalternatively be configured with substantially equally sized sides. Inthat case, illustrated in FIG. 3, a single pick-up port (e.g. port 304)is located in each corner of each side. Such an enclosure generallylimits variation in pick-up orifice liquid coverage to 12.5% to 16.7%.While the examples of FIGS. 1-3 are illustrative, other enclosureconfigurations could be envisioned, and pick-up orifices located withinthem.

At block 510, the pick-up orifice diameter is adjusted in response to acomparison of the mass flow rate through the pick-up port system ({dotover (m)}_(pu)) to the flow rate through atomizers ({dot over(m)}_(atom)). In the example of block 512, if {dot over (m)}_(pu) isless than {dot over (m)}_(atom), then the adjustment includes decreasingthe pick-up orifice diameter. That is, if the diameter of the pick-uporifices is too large, then the volume of fluid removed will consist oftoo much vapor and not enough liquid, meaning that the mass of fluidremoved is too small. Accordingly, the pick-up port orifice diametershould be decreased, thereby preventing the more easily collected vaporfrom comprising too large a portion of the total fluid volume.Conversely, in the example of block 514, if {dot over (m)}_(pu) isgreater than {dot over (m)}_(atom), then the pick-up orifice diametershould be increased. That is, if the diameter of the pick-up orifices istoo small, then the volume of fluid removed will consist of too muchliquid and not enough vapor, meaning that the mass of fluid removed istoo large. Accordingly, the pick-up port orifice diameter should beincreased.

At block 516, in one implementation, the adjustments of blocks 510-514may be performed in an iterative manner. The iterations are performeduntil adjustments made to the orifice size result the ability to removefluid from the enclosure ({dot over (m)}_(pu)) that is incrementallygreater than the fluid required by the atomizers ({dot over (m)}_(atom))to remove the system heat load.

At block 518, a heat exchanger is sized to result in completecondensation of vapor removed from all pick-up ports. In oneimplementation, a compound heat exchanger is configured to include asmany channels 146 as there are pick-up ports defined within theenclosure. In such a heat exchanger, vapor drawn through nonliquid-submerged pick-up ports can be fully condensed within associatedchannels in the compound heat exchanger. Fluid drawn throughliquid-submerged pick-up ports can be sub-cooled within associatedchannels in the compound heat exchanger. By determining the range in thequantities of liquid and vapor fluid removed from the system, the sizeof the heat exchanger can be determined. If the heat exchanger includesa separate channel to condense and/or cool fluid removed from eachorifice, then 16 channels will be required for the 16 orifices definedin the clamshell enclosure of FIGS. 1 and 2, and 24 channels will berequired for the 24 orifices defined in the enclosure of FIG. 3. Ineither case, channels within a compound heat exchanger should be sizedso that vapor drawn through a pick-up port can be fully condensed withinan associated channel.

In some implementations, the pick-up orifices can be integrated into theheat exchanger to help distribute the flow through each heat exchangerchannel. Additionally, the heat exchanger can include distributionplates consisting of a plate with orifices to distribute flow throughthe heat exchanger. Since the systems described herein are adapted forrestrictive orifices for fluid metering, they can be utilized todistribute flow in the heat exchanger. Thus, an implementation can beconfigured to replace a pick-up port comprising a single 0.025″ orificewith a pick-up port comprising five 0.011″ orifices.

At block 520, heaters for inclusion within the fluid recovery system aresized to generate sufficient vapor quality so that m_(pu) incrementallyexceeds m_(atom) upon system start-up. In general, the heaters convertliquid fluid into vapor fluid, and thereby increase the vapor quality.This increases the mass of fluid removed by pick-up orifices covered byvapor, and therefore increases the mass of fluid m_(pu) removed by thecombined number of pick-up ports. With sufficient heating at start-up,the mass of the fluid removed from the pick-up ports m_(pu) will becapable of matching the mass of fluid m_(atom) required for theatomizers.

In one example of operation of software 502 implementing the method 500,the optimum pick-up orifice diameter was found to be 0.025″. In othersystems, the orifice diameter is between 0.01″ and 0.05″. The 0.025″orifice diameter allows the system to operate from approximately 160 Wto 280 W. Below 150 W the vapor quality drops below 0.2 and the systemcannot remove enough liquid from the enclosure. Above 280 watts,temperatures within the enclosure rise above 75 C. It should be notedthat vapor quality is a dominate factor in determining if the systemwill operate. The vapor quality threshold for operation is a function ofthe hydrostatic head of the pickups. For some systems, the threshold isaround 0.23. Below this value, these systems may not operate.

In a further example of a passive fluid recovery system (e.g. systems100, 200, 300, 400 of FIGS. 1-4), the orifice size used in each of thepick-up ports should result in a balanced withdrawal fluid. That is, thefluid withdrawn from the enclosure should be balanced between the gasand liquid phases. Such balance results when fluid is removed from bothsubmerged and non-submerged pick-up ports having an orifice diameterwithin an acceptable range. In particular, the orifice diameter shouldbe adjusted so that the fluid withdrawn is balanced between liquid andgas, and so that the combined fluid mass removed results in a value ofm_(pu) that is approximately equal to, or potentially incrementallygreater than, m_(atom) in a system that is substantially attitudeindependent.

Heat Exchanger

In one implementation, a counter flow heat exchanger could be configuredwith lanced and offset folded fin stock on the ‘air side’ of the heatexchanger, and mini-channels on the ‘cooling fluid side’ of the heatexchanger. Use of folded fin stock with a tight pitch takes advantage ofthe available 1.0 pound in H₂O pressure drop on the ‘air side.’ Thisprovides a large surface area enhancement on the ‘air side’ and asatisfactory heat transfer coefficient. The implementation can includemini-channels on the liquid side. The mini-channels provide high heattransfer coefficients and a high surface area to fluidvolume-transferred ratio. Also, they prevent vapor bypass that is commonin parallel path condensers. Vapor bypass results when vapor movesrapidly through the heat exchanger by bypassing fluid in the samechannel. Use of mini-channels of a cross-sectional diameter appropriateto the fluid used, prevents vapor bypass and requires the fluid to spendthe requisite time in the heat exchanger to condense fully. While lancedand offset or folded fin stock used on the ‘wet side’ and wavy fins onthe ‘air side’ can be used to provide adequate heat transfer, other heatexchanger technologies and/or surface enhancements could alternativelyor additionally be used. For example, technologies commonly known in theart of heat exchangers and heat sinks, including pin fins, diamondpaints or foams, mini or micro-channels, etc., can be used to transferheat.

Referring again to FIG. 1, it can be seen that the vapor exiting theenclosure 102 is condensed within the heat exchanger 132 prior toentering the pump 142. If vapor is allowed to enter the pump, a loss ofpump suction will result, possibly interrupting the spray (i.e. spraydischarge from the atomizer module 106). Any pick-up port 112-126 canremove saturated vapor, saturated liquid, or any combination of the twofrom the enclosure 102. Therefore, in a preferred embodiment, eachpick-up port directs the removed fluid through a path within the heatexchanger 132 that is capable of fully condensing a stream of 100%vapor.

Example Operation of a Passive Fluid Recovery System

FIG. 6 illustrates an example method 600 by which a passive fluidrecovery system (e.g. systems 100, 200, 300, 400 of FIGS. 1-4) can beoperated. The method can be implemented by operation of a digitalcontrol system, which may be configured to execute instructions based onsoftware 602, firmware or hardware (such as an application specificintegrated circuit (ASIC)).

At block 604, a start-up sequence is performed. Blocks 606-610 arerepresentative of a start-up sequence; however, the start-up sequencecan be changed to reflect requirements of any particular passive fluidrecovery system. At block 606, the enclosed system is heated. Referringbriefly to FIG. 1, the heater 110 is configured to heat fluid containedwithin the enclosure 102, thereby reducing the quantity of liquid andincreasing the quantity of vapor (gas). In some implementations, thereare two primary reasons for this preheating. First, if the vapor quality(i.e. percentage of the fluid in the vapor phase) is too low, thenpick-up orifices exposed to vapor will remove too little fluid mass, andthe overall mass of fluid removed by all pick-up orifices will beinsufficient to provide the spray module 106 with a fluid level requiredto cool the load boards 108. A second function for the heaters 110 is topreheat the electronics (or other load 108) during cold starts inextreme environments. Accordingly, at different times during operation,the fluid heats, and also cools, the load board(s) 108. At block 608,the pump is turned on, and components in the enclosed system aresprayed. Referring to the example of FIG. 1, liquid leaving the pump 142passes through the spray module 106 and cools the load board 108. Asseen above, in some applications the spray will initially tend to warmelectronic components to more nearly their operating temperature. Atblock 610, the load boards are turned on, and the heaters (e.g. heaters110 in FIG. 1) are turned off.

At block 612, fluid is removed from the enclosure using a pump 142 and aplurality of pick-up ports 112-126 (see example of FIG. 1). Block 614shows one way that block 612 could be performed. At block 614, thepick-up ports are distributed within the enclosure 102 to includepick-up ports withdrawing liquid and pick-up ports withdrawing vapor. Inone possible implementation, as the fluid reaches a temperature setpoint (due to heating by the heater 110), the system pump is ramped upto an operational speed, wherein the pressure of fluid discharged fromthe pump reaches a steady state. Because the temperature has reached aset point, the vapor quality will have reached a level wherein pick-upports (including pick-up ports submerged in liquid and in vapor) willremove an expected and sufficient mass of vapor from the system.

At block 616, the withdrawn fluid (a mixture of liquid and vapor) iscondensed and/or cooled. For example, the fluid removed from theenclosure 102 of FIG. 1 is condensed in heat exchanger 132. In theexample of block 618, fluid removed from each pick-up port can bedirected through a channel in a compound heat exchanger. In a typicalimplementation of a compound heat exchanger, some channels would includevapor that is condensed, and other channels would sub-cool liquid.

At block 620, the fluid removed from the enclosure for condensing and/orsub-cooling is reintroduced to the enclosure. In the example of block622, liquid is typically reintroduced by spraying the liquid atelectronic components to be cooled.

CONCLUSION

Although aspects of this disclosure include language specificallydescribing structural and/or methodological features of preferredembodiments, it is to be understood that the appended claims are notlimited to the specific features or acts described. Rather, the specificfeatures and acts are disclosed only as exemplary implementations, andare representative of more general concepts.

1. A fluid recovery system, comprising: an enclosure configured tocontain fluid in both gas and liquid states; a heat exchanger,configured to receive fluid removed from the enclosure and to dischargeheat energy; a plurality of pick-up ports defined within the enclosure,wherein the heat exchanger comprises a plurality of heat exchangerchannels in which heat exchange occurs and a compound structureconfigured to include an equal number of the plurality of heat exchangerchannels and the plurality of pick-up ports, wherein each pick-up portis fluidly connected to one of the plurality of heat exchanger channels,wherein each pick-up port is associated with a separate heat exchangerchannel; wherein an orifice size used in each of the pick-up portsresults in simultaneous withdrawal of fluid from submerged andnon-submerged pick-up ports, the orifice size in each of the pick-upports sized being based at least in part on a comparison of a total massflow through pick-up ports (m_(pu)) to the total mass flow throughatomizers (m_(atom))), and a spray module, configured to re-introducecondensed fluid from the heat exchanger into the enclosure.
 2. The fluidrecovery system of claim 1, wherein the orifice size is uniformlydefined among the plurality of pick-up ports and a combined fluid massremoved from the plurality of pick-up ports is substantially attitudeindependent.
 3. The fluid recovery system of claim 1, wherein at leastone wall of the enclosure is non-planar or slanted with respect to otherwalls.
 4. The fluid recovery system of claim 1, wherein: the enclosureis configured as a clamshell; and a pair of pick-up ports are located ineach corner of the clamshell.
 5. The fluid recovery system of claim 1,additionally comprising: a heater, to increase a percentage of the fluidwithin the enclosure that is in the gas state at start-up.
 6. A methodof designing a fluid recovery system, comprising: identifying a systemheat load within an enclosure; calculating a flow rate through atomizers(m_(atom)) required to cool the system heat load; locating a pluralityof pick-up ports within the enclosure to make variation in pick-up portliquid coverage substantially attitude independent; adjusting an orificediameter on each of the plurality of pick-up ports, wherein theadjusting is in response to a comparison of a total mass flow throughpick-up ports (m_(pu)) to the m_(atom), and wherein the adjusted orificediameters result in simultaneous withdrawal of fluid from submerged andnon-submerged pick-up ports; and directing fluid withdrawn from each ofthe plurality of pick-up ports through a channel in a compound heatexchanger, having a plurality of heat exchanger channels in which heatexchange occurs, wherein the compound heat exchanger is configured toinclude an equal number of the plurality of heat exchanger channels andthe plurality of pick-up ports, wherein each pick-up port is fluidlyconnected to one of the plurality of heat exchanger channels, whereineach pick-up port is associated with a separate heat exchanger channel.7. The method of claim 6, wherein adjusting the orifice diametercomprises: if the m_(pu) is less than the m_(atom), then decreasing thepick-up orifice diameter; and if the m_(pu) is greater than them_(atom), then increasing the pick-up orifice diameter.
 8. The method ofclaim 6, additionally comprising: iterating the adjusting until them_(atom) is incrementally less than the m_(pu).
 9. The method of claim6, additionally comprising: sizing heaters for inclusion within thefluid recovery system to generate sufficient vapor quality so that them_(pu) incrementally exceeds the m_(atom) upon system start-up.
 10. Themethod of claim 6, additionally comprising: sizing channels within thecompound heat exchanger so that vapor drawn through a pick-up port canbe fully condensed within an associated channel.
 11. A method ofrecovering fluid from an enclosed system, comprising: removing fluidfrom a plurality of pick-up ports distributed within the enclosedsystem, an orifice size in each of the plurality of pick-up ports beingbased at least in parton a comparison of a total mass flow throughpick-up ports (m_(pu)) to the total mass flow through atomizers(M_(atom)), wherein the pick-up ports are distributed to include pick-upports withdrawing liquid and pick-up ports withdrawing vapor; andcondensing the withdrawn vapor, wherein condensing the withdrawn vaporcomprises directing fluid removed from each pick-up port through achannel in a compound heat exchanger, having a plurality of heatexchanger channels in which heat exchange occurs, wherein the compoundheat exchanger is configured to include an equal number of the pluralityof heat exchanger channels and the plurality of pick-up ports, whereineach pick-up port is fluidly connected to one of the plurality of heatexchanger channels, wherein each pick-up port is associated with aseparate heat exchanger channel; and reintroducing the removed fluidinto the enclosed system.
 12. The method of claim 11, wherein removingfluid from the plurality of pick-up ports comprises: removing fluidthrough two or three orifices located, generally, in each corner of theenclosed system.
 13. The method of claim 11, wherein the removing issubstantially independent of attitude of the system.
 14. The method ofclaim 11, wherein the reintroducing the removed fluid into the enclosedsystem comprises: spraying liquid at heat-producing elements within thesystem.
 15. The method of claim 11, further comprising a start-upsequence comprising: heating the enclosed system; spraying componentswithin the enclosed system; and turning on load boards containing thecomponents.
 16. A fluid recovery system, comprising: means for removingfluid from an enclosure via a plurality of pick-up ports, wherein aquantity of fluid removed from the system is substantially independentof system attitude and further wherein the quantity of fluid includesboth liquid and gas that are removed simultaneously via the plurality ofpick-up ports; means for condensing and cooling the removed fluid,wherein the means for condensing and cooling the removed fluid comprisesa compound heat exchanger comprising a plurality of heat exchangerchannels in which heat exchange occurs, each channel in communicationwith one of the plurality of pick-up ports wherein the compound heatexchanger is configured to include an equal number of the plurality ofheat exchanger channels and the plurality of pick-up ports, wherein eachpick-up port is fluidly connected to one of the plurality of heatexchanger channels, wherein each pick-up port is associated with aseparate heat exchanger channel; and means for reintroducing the removedfluid into the enclosure.
 17. The fluid recovery system of claim 16,wherein the means for removing fluid from the enclosure comprises: twoor three uniformly sized orifices defined in each corner of theenclosure.
 18. The fluid recovery system of claim 16, wherein the meansfor reintroducing the removed fluid into the enclosed system comprises:a spray module of atomizers.
 19. The fluid recovery system of claim 16,additionally comprising: means for creating vapor pressure within theenclosure prior to turning on load boards cooled by the fluid recoversystem.
 20. The fluid recovery system of claim 16, additionallycomprising; means for creating vapor pressure within the enclosureduring operation of load boards cooled by the fluid recovery system.